Coiled tubing well intervention has been known in the oil production industry for many years. A great length, often exceeding 15,000 feet of steel tubing, is handled by coiling it on a large reel, which explains the name of coiled tubing. The tubing reel cannot be used as a winch drum. The stresses involved in using it as a winch would destroy the tubing. The accepted solution in the oil industry is to pull tubing from the reel as it is required and pass it around a curved guide arch so that it lies on a common vertical axis with the well bore. To move the tubing into and out of the well bore, a device called a coiled tubing injector is temporarily mounted on the wellhead, beneath the guide arch. Examples of coiled tubing injectors include those shown and described in U.S. Pat. Nos. 5,309,990, 6,059,029, and 6,173,769, all of which are incorporated herein by reference.
Coiling tension is controlled by a tubing reel drive system and remains approximately constant no matter if the injector head is running tubing into or out of the well, or if it is pulling or snubbing. The coiling tension is insignificant by comparison to tubing weight and payload carried by the tubing in the well bore and therefore is no danger to the integrity of the tubing.
Although other methods of achieving this aim are known, injector heads used for well intervention and drilling utilize a plurality of chain loops, on which are mounted grippers for gripping the tubing. There are many examples of such injector heads. Most rely on roller chains and matching sprocket forms as a means of transmitting drive from the driving shafts to the chain loop assemblies. For the injector head to manipulate tubing, it pushes, from opposite sides, the grippers against the tubing and then concurrently moves the grippers by rotating to move the tubing in and out of the well bore.
A coiled tubing reel assembly includes a stand for supporting a spool on which tubing is stored, a drive system for rotating the reel and creating back-tension during operation of the reel, and a “level winding” system that guides the tubing as it is being unwound from and wound onto the spool. The level winding system moves the tubing laterally across the reel so that the tubing is laid across the reel in a neat and organized fashion. The coiled tubing reel assembly must rotate the spool to feed tubing to and from the injector and well bore. The tubing reel assembly must also tension the tubing by always pulling against the injector during normal operation. The injector must pull against the tension to take the tubing from the tubing reel, and the reel must have sufficient pulling force and speed to keep up with the injector and maintain tension on the tubing as the tubing is being pulled out of the well bore by the injector. The tension on the tubing is always being maintained in an amount sufficient to wind properly the tubing on the spool and to keep the tubing wound on the spool.
Although a spool can be rotated by means of a chain and a sprocket mounted on the axle of the coiled tubing spool, planetary gear drives are typically used to rotate the spool. A planetary gear drive is capable of delivering high torque at low speeds without the heaviness and expense of a chain and sprocket. Closed center planetary gear drives are usually preferred. Such drives have all of their components mounted symmetrically about the center of rotation, including the drive motor, which may be electric or hydraulic.
In a typical arrangement, the output of a planetary gear drive supports one end of the reel, connecting directly to the axle of the reel. Integral brakes are usually fitted to the planetary drive to provide a parking brake for preventing unwinding of the stored tubing when the drive motor is not powered. Planetary gearing is also referred to as epicyclic gearing. Planetary gearing comprises one or more gears, called planet gears, that revolve around a central gear called a sun gear. The planet gears are mounted to a carrier, which may rotate relative to the sun gear. An outer gearing, called an annulus, meshes with the planet gear. Planetary gearing may be either simple or compound. A simple planetary gear has one sun, one ring, one carrier and one set of planet gears. A compound planetary gearing has a more complex structure. There exist many examples of compound structures too numerous to list. In a coiled tubing reel application, the planetary gear drive functions as reduction gearing that takes a relatively high speed, low-torque input, such as from a hydraulic motor, and provides a relatively low speed, high-torque output that is coupled with the hub of the reel, with the input to the planetary gearing rotating about the same axis as its output and the spool.
A fluid swivel connects to the other end of the axle of the reel for coupling a fluid source to the coiled tubing wound on the reel. Because the planetary gear drive is connected to one end of the spool's central axle, and the fluid axle is connected to the other end, a concentric rotary union or a slip ring assembly is used to run electrical and other wires into the coiled tubing for transmitting electrical signals to and from sensors and other equipment connected to the end of the coiled tubing. The concentric rotary union must have a sufficiently large internal hub that can be bored out to pass over the axle of the reel. Alternately, to avoid having to incorporate concentric rotary ring, a chain and sprocket is used.